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Biggest Tyre On A 7.5 Inch Rim?

Captain Natro
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Captain Natro Grand Master

Captain Natro

Posted
Posted

Hey guys looking at going down to the local tyre place later in the week but thought id ask here first, i have a set of 18 inch by 7.5 inch wide rims and at the present have 235/40/18s on them. There currently fitted to an R33 and i was wondering what would be the largest tyre i could fit on this rim cause the current 235's look kinda scrawny? (guards can be rolled if rubbing occurs)

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https://www.sau.com.au/forums/topic/359284-biggest-tyre-on-a-75-inch-rim/
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jrm Veteran

jrm

Posted
Posted

Think you'll find 255/40 wouldn't be legal anyway - too big a rolling diameter change.

Pretty sure even a 245/40 would be pushing it? 245/35 should be okay though ..

From what I recall, you can't fit a tyre with a 15mm larger or smaller diameter...

sinistaGTR Grand Master

sinistaGTR

Posted
Posted

Think you'll find 255/40 wouldn't be legal anyway - too big a rolling diameter change.

Pretty sure even a 245/40 would be pushing it? 245/35 should be okay though ..

From what I recall, you can't fit a tyre with a 15mm larger or smaller diameter...

Um... a change in width has nothing to do with rolloing diameter, a change in profile does

jrm Veteran

jrm

Posted
Posted

Um... a change in width has nothing to do with rolloing diameter, a change in profile does

You seem to have a misunderstanding of how the width & profile of the tyre affects the way the tyre sits and its effective width & height and how that relates to the real-world diameter.

I'll give you a hand with a link to a tyre size calculator so that you might understand how it works:

http://www.miata.net/garage/tirecalc.html

Keep in mind that you need to keep within 15mm (larger or smaller) of the factory specification tyre/rim combination to keep legal.

Any tyre shop worth their salt should be able to advise whether a particular width/profile combination will be legal for your car (in terms of diameter-change)

sinistaGTR Grand Master

sinistaGTR

Posted
Posted

You seem to have a misunderstanding of how the width & profile of the tyre affects the way the tyre sits and its effective width & height and how that relates to the real-world diameter.

I'll give you a hand with a link to a tyre size calculator so that you might understand how it works:

http://www.miata.net/garage/tirecalc.html

Keep in mind that you need to keep within 15mm (larger or smaller) of the factory specification tyre/rim combination to keep legal.

Any tyre shop worth their salt should be able to advise whether a particular width/profile combination will be legal for your car (in terms of diameter-change)

Think you'll find 255/40 wouldn't be legal anyway - too big a rolling diameter change.

Pretty sure even a 245/40 would be pushing it?

Read your own statement how is a 255/40 and a 245/40 a different rolling diameter. If a 255/40 illegal then sure as hell a 245/40 is aswell there both the same rolling diameter....wouldnt matter if it was a 1000000/40 its still the same rolling diameter its the width that changes

jrm Veteran

jrm

Posted
Posted

Read your own statement how is a 255/40 and a 245/40 a different rolling diameter. If a 255/40 illegal then sure as hell a 245/40 is aswell there both the same rolling diameter....wouldnt matter if it was a 1000000/40 its still the same rolling diameter its the width that changes

... ? er - no - they're really not. You didn't try the tyre calculator did you.

Specification Sidewall Radius Diameter Circumference Revs/km

255/40-18 102mm 331mm 661mm 2077mm 481

245/40-18 98mm 327mm 653mm 2052mm 487

Gosh! Can you believe that - there's a difference in the side-wall despite them both being 40 profile! And wow, they're a totally different diameter by 8mm

.. maybe I'm just imagining the math being correct - or maybe you're really confused and don't understand what the numbers on tyres mean..

Let's go a step further.

Q. What is the aspect ratio? 205/65 R16 95 V

The Aspect ratio or profile is the height of the tyre form the rim to the tread area. It’s shown as a percentage of the tread width. In this case the “height” of the tyre is 65% of the width.

Wow, so the profile is actually a percentage of the tread _width_ ... wow, so that would explain why different width tyres with the same profile have a different sidewall size!

... You getting it yet?

datsun_1600 Enthusiast

datsun_1600

Posted
Posted (edited)

From Wikipedia:

"The first number in the code (e.g., "225") represents the nominal tire width in millimetres. This is followed by the aspect ratio (e.g.,"70"), which is the height of the side wall expressed as a percentage of the nominal width. "R" stands for radial and relates to the tire construction. The final number in the code (e.g.,"14") is the rim size measured in inches. The overall circumference of the tire will increase by increasing any of the tire's specifications. For example increasing the width of the tire will also increase its circumference, because the side wall height is a proportional length. Increasing the aspect ratio will increase the height of the tire and hence the circumference."

Jeff

Edited by datsun_1600
Birds Grand Master

Birds

Posted
Posted

255mm on a 7.5" wheel? What kind of dealer gives that advice? You'd need a 55 profile tyre to pull it off and that amount of sidewall would look ridiculous on an 18" wheel / probably handle like a bucket of shit. Think you should stick with 225/40 - tis a good size for 7.5". If you want bigger, get bigger wheels...you can fit 9" on a Skyline no worries.

Use this as a guide for what you can and can't fit:

http://www.federaltyres.com.au/tyres/high-performance/595

jrm Veteran

jrm

Posted
Posted

And yes, Jenna is correct, although no need to be a dick about it...there are less patronising ways to get your point across >_<

After trying so many times, and being told I'm wrong so many times - you start to get a little testy with people; I make no apologies for it.

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